This invention relates to a vapor-deposited film of selenium as a photoreceptor for electrophotography and also to a process for producing the same, characterized in that the oxygen content in the film is controlled to be not more than 50 parts per million.
Electrophotography is a photocopying process which utilizes the photoconductivity and electrostatic behavior of certain substances. Of some methods so far established, one known as the Xerox process, or xerography, involves the use of a vapor-deposited film of selenium as a photoreceptor and the transfer of image for printing as an electronic photograph. The process comprises the steps of
(a) charging, in which the surface of a photosensitive plate, consisting of an amorphous selenium layer with a high dark resistance vapor-deposited on a metal substrate, is electrostatically charged;
(b) exposure (printing), in which, upon exposure to an optical image, the areas of selenium irradiated with the light undergo a decrease in the electric resistance, the electrostatic charge escapes from the surface to the metal substrate, and the residual charge density on the photosensitive plate varies according to the amounts of exposure, thus forming an electrostatic latent image of the same pattern as the original copy on the selenium surface;
(c) development, in which a mixed powder of a toner, consisting of fine resin-coated carbon particles, and a carrier, consisting of microspheroidal glass, is dusted over the photosensitive plate surface, so that the toner adheres to the latent image and makes it visible;
(d) transfer, in which a sheet of appropriate paper is placed over the developed photosensitive plate surface, and the back of the paper is charged by the use of a corona discharge, whereby the toner on the photosensitive plate is attracted to the paper and the toner powder image is transferred onto the paper; and
(e) fixing, in which the paper onto which the image has been transferred is removed from the photosensitive plate surface and is heated by an infrared heater to fuse and deposit the toner resin permanently onto the paper.
Through a sequence of these steps a reproduced image (electronic photograph) of the original is obtained. Clearness of the copied image, or the reproducibility of the original, is largely dependent on the performance of selenium as the photoreceptor. For the evaluation of the photoreceptor performance, apparently useful measures include: (i) the corona charge characteristic which represents the electrostatic charge produced by a corona discharge of a given output; (ii) the dark decay characteristic related to the loss of charge while the photoreceptor, charged by the corona discharge, is held in the dark; (iii) charged potential decrease characteristic on exposure which represents the rate at which the charge held in darkened conditions disappears upon exposure; and (iv) residual potential which represents the potential that remains, instead of reduction to naught, after the exposure of the photoreceptor. Of these factors, the residual potential plays the most important role in stabilizing the contrast and quality of the electrophotograph and the properties of the selenium photoreceptor. In principle, the residual potential, where present, makes the contrast of the resulting electrophotograph indistinct by a so-called "ghost image" phenomenon.
As regards the residual potential, a major subject of consideration in the art has been the residual potential in the positively charged photoreceptor film, and varied attempts have been made to reduce it to zero or a minimum. For example, a known practice consists in doping a vapor-deposited film of selenium with a trace amount of a halogen, such as fluorine, chlorine, or bromine. The halogen-containing selenium film thus obtained does reduce the residual potential upon exposure to zero or a minimum. However, it undergoes such serious dark decay of the surface charge in the course of latent image formation that the functions of the resulting photosensitive plate are badly affected.
Recently, copying systems which utilize compatible positive and negative chargings have attracted attention. In this case it is ideal that the residual potentials in the positively and negatively charged film be both reduced to naught or minimized. However, the realization of the ideal is expected to involve no small difficulties. If the residual potentials upon the positive and negative charging are balanced within a not too broad range, the photoreceptor selenium film would serve satisfactorily for the copying system that depends on both positive and negative charges. For this application it is important that the photosensitive plate should not undergo serious functional deterioration, for example, worsened dark decay as is the case with the halogen doping.
In the positive-charge copying system, it is said that, even though the residual potential in the positively charged photoreceptor is reduced to a minimum or zero, a too high residual potential at the time of negative charging would have an adverse effect upon the photosensitive plate performance.